PROJECTSUMMARY PTSD is a stress-induced disorder characterized by a cluster of symptoms, including persistent, extinction- resistant memory. Behavior modification, such as exposure therapy, and pharmaceuticals have had limited success in treating PTSD, with outcomes ranging from unresponsive to an exacerbation of symptoms. As a result, there is an urgent need to develop improved therapeutics. However, therapeutic development cannot progresswithoutabetterunderstandingoftheneurobiologicalmechanismsthatunderliePTSD.Toaddressthis gap,anovelmousemodelofPTSDemployingstress-enhancedfearlearning(SEFL)willbeused.Thismodel hashighfaceandconstructvalidity,withasubgroupofmalesfromtheinbredC57BL/6Jmouselinedisplaying severalPTSD-likefeatures,including(1)extinctiondeficits,(2)feargeneralization,(3)heterogeneousresponse tostress,suchthat?resilient?and?susceptible?phenotypesdevelopwithinthesameSEFLtreatedgroup,(4) persistenceofthetraumaticmemoryformorethan30days,and(5)exaggeratedstartlereflex.Consistentwith the human literature on PTSD, these mice also show altered Fos activation in two PTSD-associated brain regions;?hypoactivationoftheinfralimbiccortex(IL)andhyperactivationinthebasolateralamygdala(BLA)with remotememoryretrieval.ItiswellestablishedthattheBLAandIL,whicharereciprocallyconnected,playavital role in the regulation of fear memories. Therefore, the central hypothesis of this proposal is that altered communicationbetweentheBLAandILunderliesthepersistent,extinction-resistantfearmemoriespresentin micetrainedinthisSEFLmodelofPTSD.GiventhatwomenaretwiceaslikelytodevelopPTSD,itisimperative thatanimalmodelsbeusedthateffectivelyrecapitulatethedisorderinbothsexes.Therefore,inthefirstaim, theSEFLprotocolwillbevalidatedinfemalesusingaseriesofbehavioralandphysiologicaltests,withthegoal ofidentifyingaprotocolthatproducesinterindividualvariabilityinstresssusceptibility,asisthecaseinmales. TheworkinghypothesisforthisaimisthatSEFLwillproduceaPSTD-likephenotypeinasubgroupoffemale mice.Inthesecondaim,theSEFLparadigmwillbeappliedtobothsexestointerrogatetheneurocircuitrythat supportsthepersistenceofextinction-resistantfearmemories.Theworkinghypothesisisthatdysregulationof celltype-specificBLAinputsontotheILsupportstress-enhanced,extinction-resistantfearmemories.Toassess the functional importance of the IL to extinction of PTSD-like memories, following identification of the cellular identityofactivatedneuronsintheILandBLA,achemogeneticapproachwillbeemployedtodeterminethe functionaleffectofmanipulatingthisregion?sactivityonfearmemoryexpressionfollowingSEFLtraining.Activity ofmonosynaptic,celltype-specificILinputswillthenbeidentifiedusingmodifiedrabiesvirus.Thisproposalis expectedtogreatlyexpandourknowledgeoftheneurobiologysupportingPTSD-likememories.